Process and apparatus for making ceramic articles

ABSTRACT

A process and apparatus for making a ceramic article, especially pipes and tubes of ceramic material such as silicon carbide, in which the article is formed by projecting a ceramic powder mixture comprising refractory particles and combustible particles against a smooth-surface metal former which is heated to a temperature of at least 850° C. where in the presence of an oxygen-rich gas the combustible particles react exothermically to produce from the refractory particles and the reaction product of the combustible particles a coherent refractory article in the shape of the former.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the formation of ceramic articles andto the apparatus used for the said formation. It is particularly, butnot exclusively, concerned with the formation of pipes and tubes ofceramic material.

2. Description of the Related Art

Ceramic pipes and tubes have traditionally been formed from clay mouldedinto the desired "green" shape and then baked in an oven. These arestill widely used but are not of high quality in regard to mechanicalstrength and ability to withstand high temperatures such that a highproportion of breakages are incurred. For ceramic pipes and tubescapable of withstanding more demanding duties, especially for such heavyduty ceramics as silicon carbide, the manufacturing methods have howeverbecome increasingly more complex. Japanese published patentspecification JP-A-03-218987 describes the manufacture of a compositesilicon carbide-metal pipe for such duties as combustion pipes of gasturbine engines. This manufacture first forms a layer of a uniformmixture of silicon carbide powder and carbon powder on the inner surfaceof a metal pipe. Excess silicon is then added and a mixed powder layerof aluminium and Fe₃ O₄ is laminated to the first layer to form atubular article. The tubular article is is then subjected to very fastrotation such that a thermit reaction occurs and creating sinteredsilicon carbide in the first layer.

The present invention has as its object the preparation of high qualityceramic articles such as pipes and tubes while avoiding such complexprocedures.

Specifically the present invention has arisen from investigations intohow to apply to the manufacture of ceramic articles a technique similarto the ceramic welding process.

In "ceramic welding" a mixture of solid refractory particles and solidcombustible fuel particles of a metal or semi-metal such as aluminiumand silicon is projected against a target surface where the combustibleparticles react in an highly exothermic manner with oxygen gas toproduce with the refractory particles a coherent refractory masstogether with the oxide(s) produced by the reaction.

There have been many patents on ceramic welding, starting with GB patent1,330,894 (Glaverbel). While it is known to employ ceramic weldingprocesses to form a refractory article, for example, a block having aparticular shape for use in constructing or repairing an industrialfurnace, it has historically mainly been used in the in-situ repair offurnace refractories, for example the refractories used in the walls ofglassmaking furnaces or coke ovens. The ceramic welding mixture isprojected to the required point of repair where it produces a coherentrefractory mass adhering to the furnace wall.

The refractory materials used in furnaces include such oxide materialsas alumina, silica and zirconia. Refractories having a base of siliconcarbide are used in certain metallurgical plant, in particular inferrous blast furnaces or in zinc distillation columns. The combustibleparticles and refractory particles used in the repair mixture areusually chosen so that the coherent refractory mass is of a similarchemical composition to the furnace refractory.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process formaking a ceramic article from a powder mixture comprising refractoryparticles and combustible particles, in which the combustible particlesare reacted exothermically in the presence of an oxygen-rich gas toproduce the article from the refractory particles and the reactionproduct of the combustible particles, characterised in that the powdermixture is projected against a metal former, which is heated to atemperature of at least 850° C., to produce a coherent refractoryarticle in the shape of the former and the former is then separated fromthe article.

Apparatus for forming a refractory mass comprising spraying means forspraying particles of oxidizable material and refractory materialtogether with oxygen are known, e.g. apparatus according to WO,A,9003848 (Willmet, Inc.). The present invention further provides apparatusfor making a ceramic article comprising projection means for a powdermixture comprising refractory particles and combustible particles,supply means for an oxygen-rich gas characterised in that it comprises ametal former made of a material which is resistant to high temperatureand to chemical attack, against which the projected particles can bereacted with the oxygen-rich gas to produce from the refractoryparticles and the reaction product of the combustible particles acoherent refractory article in the shape of the former, wherein thesurface of the metal former is free from any indentations or projectionswhich would tend to assist a bond being created between the weldmaterial and the metal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is well suited to production of all types of ceramicarticle, for example flat plates, curved plates, corrugated plates,dishes and open conduits. It is however particularly well suited to theproduction of hollow articles such as pipes, tubes, tubular connectorsand junction pieces.

The articles produced according to the invention have high levels ofrefractory properties and good mechanical strength, thus permittingtheir use in heavy duty applications of ceramic materials. They areespecially well suited for use in high efficiency heat exchangers, incorrosion-resistant immersion heaters, gas nozzles and metal ducts forexample steel ladle nozzles. The need for corrosion resistance isparticularly marked in use with molten metals such as zinc andaluminium.

The invention has the further advantage of permitting the production ofcomponents from high grade refractory material. The presence of lowmelting point phases is avoided, giving an advantage over classicaltechniques using slip casting, firing or calcining.

The invention represents a simplified alternative to the usual methodsof making ceramic articles of large dimensions or of complex shape.Moreover their production is fast and reliable, and because they arefabricated solely from the chosen starting material there isconsiderable flexibility in choice of end-product properties byappropriate choice of starting materials.

Preferred examples of the refractory particles include silicon carbide,alumina, zirconia and alumino-zirconia. Silicon carbide is especiallydesirable as a constituent of articles required to have good resistanceto high temperatures.

The combustible particles are preferably selected from silicon andaluminium or mixtures thereof.

The refractory particles preferably have a maximum dimension of 2 mm.This helps in ensuring a minimum loss of refractory by rebound from thetarget surface. The combustible particles preferably have an averageparticle diameter not greater than 50 μm. The term "average particlediameter" as employed herein denotes a dimension such that 50% by weightof the particles have a smaller dimension than this average.!

Heating of the former to a temperature of at least 850° C. ensures theignition of the combustible particles. Following ignition thecombustible particles burn with considerable release of heat. The formermay be preheated to a temperature of at least 850° C., thereby ensuringthe ignition of the combustible particles as they reach the former. Theprojection can alternatively be directed at a former which is at a lowertemperature than 850° C., for example at ambient temperature, and thento raise the temperature of the former to initiate the reaction.

The powder mixture is preferably projected from a single lance, althougha multiple lance configuration can be employed if desired.

Conveniently the carrier gas for the powder is the oxygen-rich gas usedfor the combustion. In general it is preferred to use commercial gradepure oxygen since this assists in achieving a high reaction temperature.

In general the powder is progressively applied to the former, staring atone point and working across the whole surface of the metal former. Theapplication pattern can be applied manually but conveniently the lanceand former are so mounted relative to each other that a pre-set andrepeatable mechanical movement of the lance and metal former relative toeach other is achieved.

The metal former should be made of a material which is resistant to hightemperature and to chemical attack. Stainless steel is generallypreferred. The shape of the metal former is determined by the requiredshape of the article. Thus another advantage of the invention is thatwith an appropriate choice of shape of the metal former it can be usedto produce articles of a complex configuration, for exampleheat-resistant T-piece and L-piece components used in exhaust manifolds.The surface of the former to receive the the weld material is smooth andfree from any indentations or projections so as to resist any tendencyfor a bond being created between the weld material and the metal.

Because the surface of the coherent ceramic material produced on theformer precisely matches the surface shape of the metal former, thusallowing for production of hollow articles with very precise andrepeatable internal dimensions.

The apparatus comprises means to move the former relative to theprojection lance. This can be achieved by simply moving the projectionmeans but it may be convenient for the former itself to be movable.

For pipe and tube making the metal former should be in the shape of anelongated cylinder. By rotating the cylinder about its axis the ceramicdeposit can be applied from a single lance at a single point. For pipesand tubes of length no greater than that of the cylinder the lance canbe moved parallel to the axis of the cylinder so as to cover the wholesurface of the cylinder with a ceramic coating.

A particular advantage of the process of the invention is that it can beemployed to produce tubular articles with dimensions not limited by thesize of the former. This can be achieved by removing the tubular articlefrom the cylinder as it is being formed: as the first annular band ofthe tubular article solidifies it can be gripped by a handling tool andmoved away from the cylinder in an axial direction while formation ofthe next annular band of the article is in progress. In such anarrangement the total possible length of tubing is not limited at all bythe forming process and depends on the adjacent handling space andequipment.

For many applications the separation of the article from the former canbe most easily achieved by simply withdrawing the former. Removal of theformed article from the former can be facilitated if required bypre-coating the former with a refractory cement which becomes brittle asa result of the heat applied to it during the forming procedure and theneasily breaks away from the surface of the former or the article.Alternatively the former can be cooled, for example to ambienttemperature, so that it shrinks relative to the formed article and canthereby be more readily removed.

The invention is further illustrated below with reference to thefollowing examples. It is however stressed that the invention is notlimited to the specific quantities and procedures described in theseexamples.

EXAMPLE 1

A cylindrical tube made of INOX stainless steel and rotatable about itsaxis was placed in an oven at 1000° C. for preheating. The cylinder hadan external diameter of 40 mm and a length of 150 mm and its externalsurface was smooth, having no significant indentations or projections.By means of a lance there was projected onto the pre-heated cylinderwhile it was rotated a powder mixture of the following composition (byweight):

    ______________________________________                                               Silicon carbide 79%                                                           Silicon          8%                                                           Aluminium        5%                                                           Magnesia         8%                                                    ______________________________________                                    

The silicon particles had a dimension below 45 μm and a specific surfacearea between 2,500 and 8,000 cm² /g. The aluminium particles had adimension below 45 mm and a specific surface area between 3,500 and6,000 cm² /g. The dimension of the silicon carbide particles was lessthan 1.47 mm, with 60% by weight from 1 to 1.47 mm, 20% from 0.5 to 1mm, and 20% below 0.125 mm. The magnesia particles had an averagedimension of approximately 300 μm.

The powder mixture was conveyed in a stream of commercially pure oxygen.After a few minutes a refractory layer about 10 mm thick had beenapplied to the cylinder surface. The cylinder was then cooled to ambienttemperature and was easily withdrawn to leave a formed tube of therefractory with an internal diameter of 40 mm and length of 150 mm. Theinner surface of the formed tube was completely smooth.

EXAMPLE 2

A cylindrical tube of the type described in Example 1 but of largerdimensions and with an integral heating means was coated with a layer ofrefractory cement and then heated to 1100° C. A powder mixture of thecomposition defined in Example 1 was projected in a stream ofcommercially pure oxygen from a lance onto the coated surface while thecylinder was rotated. Projection was continued for several minutes toform a refractory layer on the cement. After formation of the refractorylayer the cylinder was heating was stopped, the cylinder was allowed tocool to ambient temperature and again was easily withdrawn. In this casethe formed tube of the refractory had an internal diameter of 200 mm anda length of 1 meter.

What is claimed is:
 1. A process for making a ceramic article,comprising:a. providing a powder mixture comprised of refractoryparticles and combustible particles which react exothermically; b.heating a metal former to a temperature of at least 850° C.; c.projecting the powder mixture against the metal former, either before orafter heating the metal former to a temperature of at least 850° C., inthe presence of an oxygen-rich gas so that the combustible particlesreact exothermically and produce a ceramic article on the metal formerfrom the refractory particles and the reaction product of thecombustible particles, which ceramic article is comprised of a coherentrefractory material and has a shape which conforms to that of the metalformer; and d. separating the metal former from the ceramic article. 2.The process as claimed in claim 1, wherein the ceramic article is ahollow article.
 3. The process as claimed in claim 1, wherein therefractory particles comprise particles of at least one of siliconcarbide, alumina, zirconia and alumino-zirconia.
 4. The process asclaimed in claim 1, wherein the combustible particles are selected fromthe group consisting of silicon, aluminum, and mixtures thereof.
 5. Theprocess as claimed in claim 1, wherein the refractory particles have amaximum dimension of 2 mm.
 6. The process as claimed in claim 1, whereinthe combustible particles have an average particle diameter not greaterthan 50 μm.
 7. The process as claimed in claim 1, wherein the metalformer is preheated to a temperature of at least 850° C.
 8. The processas claimed in claim 1, wherein the powder mixture is projected from asingle lance.
 9. The process as claimed in claim 1, wherein the powdermixture is projected from multiple lances.
 10. The process as claimed inclaim 1, wherein the powder mixture is carried by a carrier gas which isthe oxygen-rich gas used for combustion.
 11. A process for making aceramic article, comprising:a. providing a metal former and pre-coatingthe metal former with a refractory cement; b. heating the metal formerto a temperature of at least 850° C.; c. providing a powder mixturecomprised of refractory particles and combustible particles which reactexothermically; d. projecting the powder mixture against the metalformer, either before or after heating the metal former to a temperatureof at least 850° C., in the presence of an oxygen-rich gas so that thecombustible particles react exothermically and produce a ceramic articleon the metal former from the refractory particles and the reactionproduct of the combustible particles, which ceramic article is comprisedof a coherent refractory material and has a shape which conforms to thatof the of the metal former; and e. separating the metal former from theceramic article, which separation is facilitated by the pre-coating ofthe metal former with the refractory cement which becomes brittle as aresult of the heat applied to it during projection so as to facilitatethe removal of the ceramic article from the metal former.
 12. Theprocess as claimed in claim 11, further comprising cooling the metalformer prior to separation of the ceramic article from the metal formerso as to facilitate the removal of the ceramic article from the metalformer.
 13. The process as claimed in claim 1, wherein the projectedpowder mixture is progressively applied to the metal former, starting atone point and working across the whole surface of the metal former. 14.A process for making a ceramic article, comprising:a. providing a powdermixture comprised of refractory particles and combustible particleswhich react exothermically; b. heating a metal former to a temperatureof at least 850° C.; c. projecting the powder mixture against the metalformer in the presence of an oxygen-rich gas so that the combustibleparticles react exothermically and produce a ceramic article on themetal former from the refractory particles and the reaction product ofthe combustible particles, which ceramic article is comprised of acoherent refractory material and has a shape which conforms to that ofthe metal former; and d. separating the metal former from the ceramicarticle, wherein the ceramic article is a tubular article which isremoved from the metal former as it is being formed.
 15. The process asclaimed in claim 1, wherein the metal former has a surface which is freefrom any indentations or projections which would tend to assistformation of a bond between the coherent refractory material and themetal former.
 16. An apparatus for making a ceramic article,comprising:projection means for projecting a powder mixture comprised ofrefractory particles and combustible particles; supply means for anoxygen-rich gas; and a metal former made of a material which isresistant to high temperature and to chemical attack, against which theprojected powder mixture can react with the oxygen-rich gas in use toproduce a ceramic article on the metal former from the refractoryparticles and the reaction product of the combustible particles, whichceramic article is comprised of a coherent refractory material and has ashape which conforms to that of the metal former from the refractoryparticles and the reaction product of the combustible particles, whereinthe metal former has a surface which is free from any indentations orprojections which would tend to assist formation of a bond between thecoherent refractory material and the metal former.
 17. The apparatus asclaimed in claim 16, wherein the projection means comprises at least onelance, and wherein the powder mixture is projected through the at leastone lance, and wherein the at least one lance and metal former are somounted relative to each other that a pre-set and repeatable mechanicalmovement of the at least one lance and the metal former relative to eachother is achievable.
 18. The apparatus as claimed in claim 17, whereinthe metal former has a shape of an elongated cylinder.
 19. The apparatusas claimed in claim 18, wherein the elongated cylinder is rotatableabout its axis.